Many ICs are made up of millions of interconnected devices, such as transistors, resistors, capacitors, and diodes, on a single chip of semiconductor substrate. It is generally desirable that ICs operate as fast as possible, and consume as little power as possible. Semiconductor ICs often include one or more types of memory, such as CMOS memory, antifuse memory, and E-fuse memory.
One-time-programmable (“OTP”) memory elements are used in ICs to provide non-volatile memory (“NVM”). Data in NVM are not lost when the IC is turned off. NVM allows an IC manufacturer to store lot number and security data on the IC, for example, and is useful in many other applications. One type of NVM is commonly called an E-fuse.
E-fuses are usually integrated into semiconductor ICs by using a narrow stripe (commonly also called a “fuse link”) of conducting material (metal, poly-silicon, etc.) between two pads, generally referred to as anode and cathode. Applying a programming current (Iprog) to the E-fuse destroys (fuses) the link, thus changing the resistance of the E-fuse. This is commonly referred to as “programming” the E-fuse. The fuse state (i.e., whether it has been programmed) can be read using a sense circuit, which is common in the art of electronic memories.
FIG. 1A is a plan view of an E-fuse 100. The E-fuse 100 has a fuse link 102 between an anode 104 and a cathode 106. The anode, fuse link, and cathode are typically polysilicon or silicided polysilicon formed entirely on relatively thick field oxide or isolation oxide. Contacts (not shown) provide electrical terminals to the anode and cathode. The fuse link has a fuse link length FLL and a fuse link width FLw. The fuse link has a relatively small cross section, which is essentially defined by the thickness of the material in which the fuse link is formed in and by the fuse link width. The fuse link width FLw is often the critical dimension (e.g., minimum polysilicon dimension) of the technology used to fabricate the IC. The small cross section of the fuse link results in Joule heating of the link during programming to convert the E-fuse to a high resistance state.
The terms “anode” and “cathode” are used for purposes of convenient discussion. Whether a terminal of an E-fuse operates as an anode or a cathode depends upon how the programming current is applied. Programming of the E-fuse can be facilitated by the physical layout. For example, the cathode 106 is larger than the fuse link 102, which generates localized Joule heating in the fuse link during programming.
During programming, current is applied through the fuse link for a specified period. The programming current heats up the fuse link more than the adjacent areas due to current crowding and differences in heat dissipation, creating a temperature gradient. The temperature gradient and the carrier flux causes electro- and stress-migration to take place and drive material (e.g., silicide, dopant, and polysilicon) away from the fuse link.
Programming generally converts the E-fuse from an original resistance to a programmed resistance. It is desirable for the programmed resistance to be much higher (typically many orders of magnitude higher) than the original resistance to allow reliable reading of the E-fuse using a sensing circuit. A first logic state (e.g., a logical “0”) is typically assigned to an unprogrammed, low-resistance (typically about 200 Ohms) fuse state, and a second logic state (e.g., a logical “1”) to the programmed, high-resistance (typically greater than 100,000 Ohms) fuse state. The change in resistance is sensed (read) by a sensing circuit to produce a data bit.
FIG. 1B is a side view of the E-fuse 100 of FIG. 1A. The E-fuse 100 is fabricated from a layer of link material 101 that is deposited on the IC substrate and patterned using photolithographic techniques to define the anode 104, cathode 106, and fuse link 102. The fuse link 102 has a fuse link thickness FLT that is essentially the thickness of the layer of link material 101. The E-fuse is on field oxide 108 that is formed on semiconductor material 110 (e.g., silicon).
E-fuse elements are particularly useful due to their simplicity, low manufacturing cost, and easy integration into CMOS ICs using conventional CMOS fabrication techniques. Conventional programming techniques use an on-chip current generator and an external resistor to set the desired programming current level. However, incorrect programming current can result in improperly programmed bits, and correct programming current is critical in obtaining high programming yield. Incorrect (high) programming current can also cause physical damage to structures near the E-fuse. Other problems arise when ICs are scaled to smaller design geometries (node spacings) because the programming conditions for one design geometry might not be optimal for another design geometry, undesirably reducing programming yield or increasing programming time. It is desirable to provide E-fuse techniques that overcome the problems of the prior art.